Method for the fabrication of multipole magnets

ABSTRACT

A method of forming a multipole magnet wherein two backing sheets of paper or film material are screen or stencil printed with an ink containing magnetic particles and a carrier material to provide respective complementary patterned areas thereon, the patterned areas of ink are cured or dried, the respective ink patterns are magnetized using a magnetic field perpendicular to a plane of the backing sheets, and the sheets are combined into a laminate wherein the patterned areas are located between the two sheets.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the fabrication ofmultipole magnets and, in particular, to a method for the fabrication ofmultipole magnets having a crystalline phase of an alloy of Fe, B and R,where R is a rare earth element.

2. The Prior Art

Magnetic materials and permanent magnets are important materials whichare used in many applications, including electrical appliances andelectronic devices. In view of the increasing requirement forminiaturization and the greater demands placed on electrical appliancesand electronic devices, there has been an increasing demand for improvedmagnetic materials and permanent magnets.

EP-A-0101552 describes magnetic materials based on alloys of the typeFe--B--R containing at least one stable compound of the ternary Fe--B--Rtype, where R is a rare earth element including yttrium, which compoundcan be magnetized to become a permanent magnet. The amount of rare earthR is generally in the range of from 8 to 30 atomic percent.

EP-A-0108474 describes a magnetically hard alloy composition comprisingat least 10 atomic percent of one or more rare earth elements, 0.5 to 10atomic percent of boron; and iron or mixtures of iron with a transitionmetal element, the alloy containing a major portion of magneticallyhard, fine crystallites having an average diameter of less than 400nanometers.

With the development of the rare-earth magnets, one of the majorbenefits is the high coercivity, or resistance to demagnetization. Thismeans that a high applied magnetic field is required before anypermanent damage to the magnet can occur. This is particularly relevantto magnets which are very thin (in the direction of magnetism), andalready have a significant internally generated demagnetizing field dueto the aspect ratio. Many applications now demand (or would benefitfrom) magnetic components that are very thin, due to constraints of sizeand weight, and can only be addressed by utilizing the high coercivityof these types of magnet.

Problems arise in respect of the magnetization of these materials astheir inherently high magnetic stability means that very high externalfields must be applied to achieve a high percentage of the availablemagnetic strength (saturation). For isotropic materials such as certainmelt spun NdFeB alloys the situation is even worse because themagnetization process is attempting to magnetize the majority of grainsin the material in a direction which is not a preferred or "easy"direction. Thus, in order to achieve a magnetization which is within afew percent of the saturation value, external fields of three to fourtimes the intrinsic coercivity are required. The normal method ofmagnetizing these high coercivity materials is to discharge a very largecurrent, from a bank of capacitors, through a copper coil or arrangementof copper wires. The cross sectional area of the copper wire obviouslyhas to be large enough to prevent melting of the copper by resistanceheating. It is not unusual for currents in excess of 10,000 amps to berequired in order to generate sufficiently high magnetizing fields.

Apart from the fact that these materials are difficult to magnetize, thetypes of application for which they are required are becoming moredemanding. For example, one of the major markets for these materials isin the permanent magnet components for motors. Depending on the designof the system these may be a number of separately magnetized componentsthat are assembled in a circular arrangement or a number of magnetized"poles" may be imprinted around the circumference of a continuous ringsample. The latter is becoming the favored route as this reduces thedifficulty and costs of assembly.

As motors become smaller (i.e. smaller rings) and the number of polesrequired increases, the difficulty of imprinting the required patternincreases, due to the physical volume of the copper windings required togenerate the high fields.

The combination of materials which are difficult to magnetize, smalldiameter components and more complex magnetization patterns (i.e. largernumbers of poles) all combine to limit the capability of the existingcapacitor discharge technology. A further limitation is the precision ofthe transition between the imprinted poles. For devices such as steppermotors, which rely greatly on the accuracy of the imprinted pattern andthe width of the transition for precision movement, there is a need tocontrol all aspects of the magnetization pattern; something which isexceedingly difficult to do with conventional magnetization techniques.The need for high definition of poles in all motor systems becomes moreimportant as the efficiency of designs improves.

We have now developed a method for the production of multipole magnetswhich avoids the disadvantages of the prior art method discussed above.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method for the fabricationof multipole magnets, which method comprises the steps of

i) screen or stencil printing an ink comprising a mixture of particlesof a magnet material and a carrier material onto a backing sheet in apredetermined pattern;

ii) curing or drying the patterned sheet and magnetizing the areasforming the predetermined pattern in step (i) using a magnetic fieldperpendicular to the plane of the backing sheet to produce North poleson one side of the sheet and South poles on the other side of the sheet;

iii) screen or stencil printing an ink comprising a mixture of a magnetmaterial and a carrier material onto a backing sheet in a pattern whichis complementary to the pattern in step (i), the ink being printed inthose areas where no ink was printed in step (i)

iv) curing or drying the patterned sheet and magnetizing the areasforming the complementary pattern in step (iii) using a magnetic fieldperpendicular to the plane of the backing sheet to produce North poleson one side of the sheet and South poles on the other side of the sheet;

v) turning over the sheet from step (iv); and

vi) bringing the sheet from step (ii) and the reversed sheet from step(v) together to form a laminate, thereby forming a multipole magnet.

The backing sheet which is used in the method of the present inventionmay be paper or a film material. Optionally, the paper or film materialmay be coated with a release agent.

The screen or stencil printing may be carried out according to wellknown techniques, using conventional screen or stencil printingequipment, using either a screen, where the mesh size is large enough toavoid clogging by the magnet particles, or more preferably (especiallyfor relatively thick printed layers) a simple metal stencil typically ofthe order of 0.1 to 2 mm thick. The screen and stencil designs areproduced by conventional methods.

While the magnet material may be any permanent magnetic material, it ispreferred that it is an alloy of the type Fe--B--R, where R is a rareearth metal, preferably neodymium. The magnet material will generally bein the form of fine particles of a magnet powder, preferably the powderhaving a particle size of less than 100 micrometers. The carriermaterial is preferably a low viscosity carrier material, typicallyhaving a viscosity in the range of from 10 to 1000 centipoise. Thecarrier material may be, for example, an epoxy resin, either as a liquidresin or dissolved in suitable solvents, an air activated or chemicallyactivated silicone rubber, a cyanoacrylate, polyvinyl alcohol, athermosetting or thermoplastic polymer, or a low melting point metal.Epoxy resin based carrier materials are generally preferred for use inthe invention.

The magnet powder is generally contained in the magnet ink in an amountof from 10 to 60% by volume of the ink composition. Preferably themagnet ink is screen printed onto the backing sheets in steps (i) and(iii) of the method of the invention and, accordingly, the ink shouldhave a suitable viscosity for appropriate printing.

After the backing sheets have been printed in steps (i) and (iii), thepatterned sheets are cured or dried prior to the magnetization step. Forexample, if an epoxy resin is used as the carrier material, then it canbe cured to give a semi-rigid structure.

The magnetization which is carried out in steps (ii) and (iv) may becarried out using a simple magnetization field perpendicular to theplane of the backing sheet, to produce North poles on one side of thesheet and South poles on the other side of the sheet. No complicatedequipment is required to effect such magnetization.

As previously indicated, the pattern which is produced in step (iii) ofthe method of the invention is complementary to the pattern produced instep (i) so that when the pattern sheet from step (iv) is turned over,it will match with the pattern produced in step (ii) to form a multipolemagnet.

One or both of the sheets produced in steps (ii) and (iv) may be coatedwith an adhesive prior to the lamination in step (vi) of the method ofthe invention. The lamination will generally be carried out under heatand/or pressure in order to produce a stable structure. The backingsheet upon which the patterns are printed may be retained on either sideof the laminate to provide physical support and protection, or if thebacking sheets are coated with a suitable release agent they may beremoved, thereby minimizing air gaps in the finished assembly.

Using the method of the present invention a strip of multipole magnetsis produced which may then be wound around a circular former to producea rotor assembly. Alternatively, the multipole magnet may be used in itslinear form in a sensor or actuator.

The patterns which are produced in steps (i) and (iii) of the presentinvention may be simple patterns of a bar-type format, or may be morecomplex patterns which are impossible to produce by conventionalmagnetization methods. Furthermore, several different magnetizationpatterns can be combined into a single component, if desired. Forexample, offset patterns may be produced, such patterns being of use instepper motors. Alternatively, graduated spacing may be provided forposition sensors, or herringbone patterns produced for photocopierrollers. The definition between the poles of the multipole magnet ishigh using the technique of the present invention and the spacingbetween the poles is limited only by the resolution of the printingtechnique. Furthermore, the size of the component is limited only by thesize of the printing equipment. Using the method of the presentinvention very thin multipole magnets, of the order of 0.2 mm inthickness, can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described with reference to theaccompanying drawings, in which;

FIG. 1 is a diagram of a typical multipole motor ring;

FIGS. 2a and 2b are a plan view and side elevation, respectively, of abacking sheet printed with a predetermined magnetic pattern, aftermagnetization;

FIG. 3 illustrates two components printed with predetermined patterns,after magnetization, ready for lamination; and

FIG. 4 illustrates the two components of FIG. 3 laminated together;

FIG. 5 illustrates an offset pattern printed on one component, forproportional control in stepper motors;

FIG. 6 illustrates a herringbone pattern printed on one component, foruse in photocopier rollers.

FIG. 7 illustrates a print pattern produced using a stencil mask asdescribed in Example 1;

FIGS. 8, 9 and 10 illustrate the measured results for the printedmagnets produced according to Example 1 at total effective airgaps of0.7, 1.25 and 2.225 nm; and

FIGS. 11, 12 and 13 illustrate the calculated effective value fromfinite models for the magnets of Example 1 at total effective airgaps of0.75, 1.25 and 2.25 mm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 illustrates a typical multiple motorring comprising a plurality of magnetic sections 1. Each magneticsection 1 comprises either a South pole at the exterior surface of thering or a North pole at the exterior surface of the ring, the North andSouth poles alternating around the ring. Each segment then presents theopposite pole to the inner surface of the ring, i.e., a segment whichhas a South pole facing outwards will have a North pole facing inwards.

Referring to FIGS. 2a and 2b, a backing sheet 2 has a plurality ofsegments 3 of a magnet ink printed thereon. The printed segments 3 arearranged in a predetermined pattern and are magnetized, by means of amagnetic field, to produce a plurality of North poles facing upwardly asviewed in FIG. 2b and a plurality of South poles facing downwardly.

Referring to FIG. 3, a backing sheet 2 with its predetermined pattern ofprinted segments 3 has positioned thereabove a complementary sheet 4with segments 5 printed in a predetermined pattern thereon. The printedsegments 5 on backing sheet 4 are magnetized so that the North poles ofthe magnets face downwards and the South poles of the magnets faceupwards, as viewed in section in FIG. 3. Thus, the printed pattern onbacking sheet 4 is complementary to that on backing sheet 2.

FIG. 4 illustrates the laminate produced by laminating backing sheets 2and 4 together. It will be noted that in the finished tape, segments 3alternate with segments 5 along the length of the tape. The backingsheets 2 and 4, if coated with a release material, may be removed fromthe laminated assembly, if desired.

If adhesion of the two components during the lamination process is noteffected by further curing of the binder, then prior to the laminationof sheet 4 to sheet 2, one or both of the sheets is sprayed with asuitable adhesive and the lamination is then effected using heat and/orpressure.

The laminated assembly illustrated in FIG. 4 may be used to form amultiple rotor as shown in FIG. 1.

FIG. 5 illustrates one component of a laminate in which an offsetpattern is printed to provide proportional control in stepper motors.The areas 7 and 8 are printed with a magnet ink in a regular pattern,but with a different pitch. The two components are then laminated in thesame way as described above. The spacings between the patterned areas 7and 8 are adapted to receive a complementary pattern printed on a secondcomponent (not shown) to form a laminate.

FIG. 6 illustrates one component of a laminate in which a herringbonepattern is printed as areas 9. The spacings between the areas 9 areadapted to receive a complementary pattern printed on a second component(not shown) to form a laminate.

The present invention will be further described with reference to thefollowing Example.

EXAMPLE 1

Printed magnet patterns were produced by screen printing a magneticpaste through a stencil mask consisting of a series of slots; 1 mm wide,5 mm long, and spaced evenly 1.5 mm apart, cut into 0.25 mm thicknessstainless steel, to produce the print pattern as shown in FIG. 7.

The magnetic paste was prepared from a 50 volume loading of MagnequenchMQP-D NdFeB powder, <63 μ, in size, dispersed in a slow curing, lowviscosity liquid epoxy resin system.

The paste was stencil printed by hand onto a 0.1 mm thick polyster sheetsubstrate and the printed sheets were then cured at 100° C. for 30-60minutes before magnetization.

The printed magnet patterns were magnetized by placing the printedsubstrate between the closely spaced poles (approx. 5 mm pole gap) of anelectromagnet and applying full power (120V, 12.5A) for 10 seconds.

The printed strips were thus magnetized normal to the substrate. All theprint patterns were aligned the same way between the magnetizing polessuch that the printed side gave a nominal "north" for the direction ofmagnetization (FIGS. 2a, 2b).

Pairs of printed pattern, were then laminated together, printed sideinwards, to form an alternating pattern of opposite poles on suchsurface, with the substrate forming the outermost layers, as shown inFIGS. 3 and 4.

In practice, due to the slight "slumping" of the magnetic paste duringprinting and curing, the separation between printed bars needs to beslightly larger than their width in order that the two layers caninterlock. In this example, bars of 1 mm width were printed onto eachsubstrate with 1.5 mm separation between bars. By control of thedistance between printed magnets on either of the two substrates theradius of curvature for an interlocked pattern can be controlled,allowing its use on, for example, a cylindrical rotor.

Magnetic "profile" measurements were carried out using a three axis(XYZ) table fitted with a high sensitivity Linear Hall Effect sensor(type UGN3503) at the base of the moving arm. Both positioning andmagnetic field strength measurements of the sensor were carried out bycomputer control.

The results obtained for an interlocking pattern of 1 mm wide poles,with alternating north and south transitions, give good agreement with afinite element model, and shown a regular sinusoidal magnetizationpattern.

The measured results are shown in FIGS. 8, 9 and 10 for the interlockedarrangement produced above at increasing total effective airgaps (TEAGS)of 0.75, 1.25 and 2.25 mm, respectively. The TEAG is the distance fromactive element of the sensor and the printed magnet surface. At thesubstrate surface, corresponding to a TEAG of effective airgap of 0.75mm (FIG. 8) the field varies sinusoidally along the measurement pathwith peak values of ±20 mT (±200 gauss) at the center of eachalternating pole. At larger TEAGS of 1.25 and 2.25 mm (FIGS. 9 and 10)the measured field levels fall off.

The results for FIGS. 8, 9 and 10 are in good agreement with thecalculated values from the finite element model for TEAGS of 0.75, 1.25and 2.25 mm shown in FIGS. 11, 12 and 13, respectively.

We claim:
 1. A method for the fabrication of multipole magnets, whichmethod comprises the steps of:i) using a technique selected from screenand stencil printing to print an ink comprising a mixture of particlesof a magnet material and a carrier material onto a first backing sheetin a predetermined pattern; ii) subjecting the patterned first backingsheet to a process selected from curing and drying and magnetizing theareas forming the predetermined pattern in step (i) using a magneticfield perpendicular to the plane of the first backing sheet to produceNorth poles on one side of said first backing sheet and South poles onthe other side of said first backing sheet; iii) using a techniqueselected from screen and stencil printing to print an ink comprising amixture of a magnet material and a carrier material onto a secondbacking sheet in a pattern which is complementary to the pattern in step(i), the ink being printed in those areas where no ink was printed instep (i); iv) subjecting the patterned second backing sheet to a processselected from curing and drying and magnetizing the areas forming thecomplementary pattern in step (iii) using a magnetic field perpendicularto the plane of the second backing sheet to produce North poles on oneside of said second backing sheet and South poles on the other side ofsaid second backing sheet; v) turning over said second backing sheetfrom step (iv) to become reversed; and vi) bringing said first backingsheet from step (ii) and the reversed second backing sheet from step (v)together to form a laminate, thereby forming a multipole magnet.
 2. Amethod as claimed in claim 1, wherein the first and second backing sheetis selected from the group consisting of paper and a film material.
 3. Amethod as claimed in claim 1, wherein the magnet material is an alloy ofthe type Fe--B--R, where R is a rare earth metal.
 4. A method as claimedin claim 3, wherein the rare earth metal is neodymium.
 5. A method asclaimed in claim 1, wherein the magnet material has a particle size ofless than 100 micrometers.
 6. A method as claimed in claim 1, whereinthe carrier material is selected from the group consisting of an epoxyresin based carrier material, an air activated silicone rubber, achemically activated silicone rubber, a cyanoacrylate, apolyvinylalcohol, a thermosetting polymer, a thermoplastic polymer, anda low melting point metal.
 7. A method as claimed in claim 1, wherein atleast one of the first and second backing sheets produced in steps (ii)and (iv) is/are coated with an adhesive prior to the lamination in step(vi).
 8. A method as claimed in claim 1, wherein the first and secondbacking sheets are coated with a release agent and are removed after theformation of the laminate in step (vi).
 9. A method as claimed in claim1, wherein the ink is printed onto the first and second backing sheetsin steps (i) and (iii), respectively, using a technique selected fromscreen and stencil printing.